The present invention relates to a magnetic memory configuration for storing data.
A magnetic memory such as a magnetic random access memory (MRAM) is a nonvolatile memory for the long-term storage of data.
A typical magnetic memory configuration, as is shown for example in FIG. 3, contains a memory cell array in which the individual memory cells are disposed in a matrix form. Word lines extend along the rows of the memory cell array, and bit lines extend along the columns of the memory cell array. The memory cells, in which the information is stored, are situated at the crossover points of the individual word and bit lines.
A magnetic memory cell usually has a construction in which two ferromagnetic layers are separated by a nonmagnetic layer. The magnetic field in one ferromagnetic layer (a magnetically hard layer) is fixed, while the direction of the magnetic field in the other ferromagnetic layer (a magnetically soft layer) can be set parallel or antiparallel thereto. These two stable orientations, parallel and antiparallel, represent the logic values xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d in the storage of information items.
The direction of the magnetic field in the soft layer in a selected memory cell can be changed by applying a current to a word line and a bit line which cross at the memory cell. The currents generate magnetic fields which, if they are combined, can change over the magnetization direction of the soft layer of the selected memory cell from parallel to antiparallel, or vice versa. A magnetic field that does not suffice for changing over the magnetization direction in the soft layer acts on all the other memory cells along the word and bit lines that cross at the selected memory cell.
In this case, in customary memory cells, the direction of the current flowing through the word line is always the same, while the direction of the current flowing through the bit line is altered depending on the information to be written.
FIGS. 2A and 2C illustrate the magnetic fields caused by the respective currents in a conventional memory configuration, while FIGS. 2B and 2D illustrate the orientation of the magnetic fields in the ferromagnetic layers. In FIGS. 2B and 2D, reference symbol 1 in each case designates the hard magnetic layer, and reference symbol 2 in each case designates the soft magnetic layer.
In FIG. 2A, the direction of the current flowing through the bit line corresponds to a logic xe2x80x9c0xe2x80x9d, while it corresponds to a logic xe2x80x9c1xe2x80x9d in FIG. 2C. In this case, WL indicates the magnetic field generated by the current flowing through the word line, while BL0 and BL1 respectively indicate the magnetic field generated by the current flowing through the bit line. H0 and H1 in each case indicate the magnetic field resulting from the superposition of the two magnetic fields.
Consequently, a parallel state of the magnetic fields in the two layers is present in FIG. 2B, while an antiparallel state of the magnetic fields in the two layers is present in FIG. 2D.
In accordance with the orientation of the magnetic field in the soft layer of the selected memory cell, the memory cell has different resistances perpendicular to the layer planes. The information stored in a memory cell can thus be read out by determining the resistance perpendicular to the layer planes.
Impairments brought about by ageing are problematic in magnetic memory cells. An example is the occurrence of the effect wherein after relatively long use, non-selected memory cells are also changed over by a current being switched-on on the word line and do not return to their initial position again after the current has been switched-off. The effect wherein memory cells to which a xe2x80x9c1xe2x80x9d has been multiply written still store a xe2x80x9c1xe2x80x9d in the event of a xe2x80x9c0xe2x80x9d being written is likewise known. Furthermore, it is known that after relatively long use, even the magnetization direction of the magnetically hard layer can be altered, which likewise leads to erroneous information storage.
U.S. Pat. No. 6,111,783 discloses a magnetic memory cell configuration in which, when writing information, the intensity of the current through the word lines is greater than the intensity of the current through the bit lines. On the one hand this prevents the magnetic field in the soft layer of non-selected memory cells from being changed over, and on the other hand this also reduces the energy consumption of the memory cell.
It is accordingly an object of the invention to provide a magnetic memory configuration that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the ageing phenomena mentioned above are reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a magnetic memory configuration. The magnetic memory configuration contains a cell array having magnetic memory cells disposed along a first direction and a second direction crossing the first direction, a multiplicity of electrical lines disposed along the first direction, and a multiplicity of electrical lines disposed along the second direction. The magnetic memory cells are in each case disposed at crossover points of the electrical lines. A first current supply device is provided and supplies respectively selected electrical lines along the first direction with a first current. The first current supply device is configured for changing over a direction of the first current. A second current supply device is provided and supplies respectively selected electrical lines along the second direction with a second current. The second current supply device sets a direction of the second current in accordance with an information item to be written.
Through a simple change in the direction of the current flowing through the word line, it is possible to avoid the effect wherein non-selected memory cells are changed over by the magnetic field of the word line.
Normally, the soft magnetic layer of the memory cell is deflected from its stable position by the magnetic field caused by the current flowing through the word line and is then switched either into the opposite stable position or into its initial position by the magnetic field caused by the current flowing through the bit line. In cells which see a bit line field, i.e. a switching field, it is expected that although they are deflected (enabled) by the applied word line field, they then switch back into their initial position again. Upon multiple repetition of this process it can happen that not all the domains then return to their initial position. This effect is avoided by rotating the direction of the current flowing through the word line.
A further advantage of the present invention is that electromigration effects can be avoided by the arbitrary changeover of the direction of the current through the word line. In the case of aluminum interconnects, in particular, a material migration occurs in interconnects under the influence of an electric current flow. The material transport is physically caused by collisions between moving electrons and the positive metal ions of the crystal lattice. Consequently, the material transport always takes place in the direction of the electron flow and counter to the technical current direction. If the current direction is reversed relatively often as in the case of the present invention, then this material migration, which might otherwise lead to an interruption of the interconnect, can be avoided.
In a development of the concept of the invention a plurality of layers of memory cells may be disposed one above the other. A first memory cell is disposed above a line running in the first direction. Disposed above that is a line running in the second direction. Situated above that in turn is a further memory cell, above which is disposed a line running in the first direction. The line disposed between the two layers of magnetic memory cells is connected to the current supply device that changes over the direction of the current. In this case, the central line plane undertakes the enable function for the memory cell layer disposed above and below it.
In accordance with an added feature of the invention, the first current supply device contains a counting device for counting accesses to one of the electrical lines in the first direction and reverses a current direction after a predetermined number of accesses to the one electrical line in the first direction for an next access.
In accordance with a further feature of the invention, the electrical lines along the first direction are word lines, and the first current supply device, for each one of the word lines along the first direction, contains in each case two inverters having outputs and inputs. Each of the word lines is disposed between the outputs of the two inverters. The first supply device further has a control device for feeding in each case one of two logic levels to the inputs of the two inverters depending on a desired current direction.
In accordance with an additional feature of the invention, the first current supply device for each of the electrical lines along the first direction, contains in each case a first and a second voltage source, at least two selection transistors with controlled paths, and a control device controlling the first and second voltage sources such that the first voltage source provides a high voltage signal and the second voltage source provides a low voltage signal (and vise versa) which are applied to the controlled paths of the selection transistors.
In accordance with another feature of the invention, the electrical lines along the first direction are word lines, and the controlled paths have terminals and one of the word lines is connected between the terminals of the controlled paths of the selection transistors.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a magnetic memory configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.